The Long Count Calendar of the Ancient Mayans ends on Dec 21 2012. There isn't much information regarding what the Mayans thought would occur in 2012, but the consensus of opinion is that there will be great change. To some people this means a positive, spiritual change. Others, like Robert Bast, consider that a catastrophic event may have been predicted.

There is zero scientific evidence that anything will happen, but there are numerous things that could easily threaten the human species in the year 2012, such as a geomagnetic reversal, asteroid strike or supernova. Or something more intimate, like a flu pandemic, or a nuclear war.